A preparation method of a lithium iron phosphate positive electrode taking a bio-based cellulose as a carbon source

Abstract

This invention relates to the field of battery materials technology, specifically to a method for preparing lithium iron phosphate cathodes using bio-based cellulose as a carbon source. This method solves the problems of high internal resistance and high heat generation in conventional lithium iron phosphate batteries prepared from existing lithium iron phosphate materials. The method utilizes a mixture of glucose and bio-based cellulose as a carbon source, which is milled, dried, and calcined to form primary carbon-coated lithium iron phosphate. Then, bio-based cellulose is used as a modifier, and the mixture is milled, dried, and calcined again to form secondary carbon-coated lithium iron phosphate. Finally, the secondary carbon-coated lithium iron phosphate is used as the active material to obtain the lithium iron phosphate cathode. This method uses a secondary calcination process to effectively suppress particle agglomeration, resulting in a more uniform carbon coating layer. The selection of a carbon source with significant differences in carbonization temperature helps to form a continuous, disordered-graphitized carbon layer structure during the secondary carbonization process, enabling the lithium iron phosphate material to exhibit excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a method for preparing lithium iron phosphate cathodes using bio-based cellulose as a carbon source. Background Technology

[0002] Lithium iron phosphate (LFP) is an electrode material for lithium-ion batteries, with the chemical formula LiFePO4 (LFP). It is primarily used in various lithium-ion batteries. LFP has good lattice stability; the insertion and extraction of lithium ions have little impact on the lattice, resulting in good reversibility. Therefore, it can be charged and discharged more than 2000 times under 100% DOD conditions. However, conventional LFP batteries have high internal resistance and generate a lot of heat. The high internal resistance of lithium iron phosphate itself leads to greater heat generation at the same current, especially in large single-cell batteries. If the heat dissipation system is inefficient, the heat generation problem will be exacerbated. To improve conductivity, conductive carbon materials are often incorporated.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a method for preparing lithium iron phosphate cathode using bio-based cellulose as a carbon source, which solves the problems of high internal resistance and high heat generation in conventional lithium iron phosphate batteries prepared from existing lithium iron phosphate.

[0005] The objective of this invention can be achieved through the following technical solutions: In a first aspect, this application provides a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step A1: Add lithium carbonate, iron phosphate, carbon source and deionized water to a mixer and stir for 20-40 minutes at 30-60℃ and 800-1000 r / min. Then place it in a sand mill and mill for 1-2 hours at 1000-1500 r / min, and then mill for 3-4 hours at 3000-4000 r / min to obtain a first-stage milled slurry. Step A2: Spray dry the primary sand-milling slurry at an inlet temperature of 170-180℃ and an outlet temperature of 110-120℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step A3: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at a temperature of 400-1000℃ for 5-10 hours to obtain primary carbon-coated lithium iron phosphate. Step A4: Add the primary carbon-coated lithium iron phosphate, bio-based cellulose, and deionized water to a sand mill and mill for 30-50 minutes at a speed of 1000-1500 r / min. Then mill for 1-3 hours at a speed of 3000-4000 r / min to obtain a secondary milled slurry. Step A5: Spray dry the secondary sand-milling slurry at an inlet temperature of 170-180℃ and an outlet temperature of 110-120℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step A6: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at a temperature of 400-1000℃ for 5-10 hours to obtain secondary carbon-coated lithium iron phosphate. Step A7: Mix the secondary carbon-coated lithium iron phosphate, polyvinylidene fluoride, acetylene black and N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 110°C for 1 hour. After that, roll and cut it, and then place it in an oven to dry at 110°C for 12 hours to obtain the lithium iron phosphate cathode.

[0006] In a preferred embodiment of the present invention, the ratio of lithium carbonate, iron phosphate, carbon source and deionized water in step A1 is 3000g:752.1g:330g:10205.3mL.

[0007] In a preferred embodiment of the present invention, the carbon source in step A1 is one of glucose and bio-based cellulose, or a mixture of both in any proportion.

[0008] In a preferred embodiment of the present invention, the ratio of primary carbon-coated lithium iron phosphate, bio-based cellulose, and deionized water in step A4 is 3818.1g:80g:9545.25mL.

[0009] In a preferred embodiment of the present invention, step A7: the ratio of the amount of secondary carbon-coated lithium iron phosphate, polyvinylidene fluoride, acetylene black and N-methylpyrrolidone is 800g:100g:100g:1050g.

[0010] In a preferred embodiment of the present invention, the bio-based cellulose is prepared by the following steps: Step B1: Place wheat bran in an oven and dry it at a temperature of 60-80℃ for 12 hours to obtain dried wheat bran. Then grind it through a 100-300 mesh sieve to obtain wheat bran powder. Step B2: Add wheat bran powder to sodium hydroxide solution and stir the reaction at a temperature of 60-100℃ and a stirring rate of 200-500r / min for 1.5-5h. After the reaction is completed, cool the reaction product to room temperature, then filter it under vacuum. Wash the filter cake with distilled water 3-5 times and then freeze-dry it to obtain wheat bran cellulose. Step B3: Add wheat bran cellulose to sodium hydroxide solution and stir the reaction at a temperature of 30-40℃ and a stirring rate of 200-500 r / min for 3-8 hours. After the reaction is completed, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step B4: Add the alkalized cellulose solution to the reaction vessel, introduce chloromethane, and react for 4-6 hours at a temperature of 40-50℃ and a pressure of 0.3-0.5MPa. Then add propylene oxide and continue the reaction for 2-3 hours at a temperature of 70-80℃ and a pressure of 0.6-0.8MPa. After the reaction is completed, cool the reaction product to room temperature, then adjust the pH to 6.5-7.5 with acetic acid solution, then vacuum filter, wash the filter cake 3-5 times with ethanol solution, freeze dry, and pulverize through a 100-300 mesh sieve to obtain bio-based cellulose.

[0011] In a preferred embodiment of the present invention, the ratio of wheat bran powder to sodium hydroxide solution in step B2 is 10g:100-150mL; the mass fraction of the sodium hydroxide solution is 5-10%.

[0012] In a preferred embodiment of the present invention, the ratio of wheat bran cellulose to sodium hydroxide solution in step B3 is 10g:80-100mL; the mass fraction of the sodium hydroxide solution is 20-30%.

[0013] In a preferred embodiment of the present invention, the ratio of alkalized cellulose solution, chloromethane and propylene oxide in step B4 is 100 mL: 20-60 mmol: 15-45 mmol.

[0014] In a preferred embodiment of the present invention, the mass fraction of the acetic acid solution in step B4 is 8-10%.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention discloses a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source. The method utilizes a mixture of glucose and bio-based cellulose as the carbon source, followed by milling, drying, and calcination to form primary carbon-coated lithium iron phosphate. Then, bio-based cellulose is used as a modifier, and the mixture is milled, dried, and calcined again to form secondary carbon-coated lithium iron phosphate. Finally, the secondary carbon-coated lithium iron phosphate is used as the active material to obtain the lithium iron phosphate cathode. The bio-based carbon source possesses good conductivity, low cost, and a unique microporous structure that is conducive to the production of lithium iron phosphate. + The transport-promoting effect. Specifically, the microporous structure of this carbon source can effectively optimize the electrolyte penetration pathway, thereby accelerating the Li-ion transport process. + The diffusion rate within the electrode material is crucial for improving the electrochemical performance of the battery. Furthermore, using biochar as a carbon coating material not only possesses unique environmentally friendly characteristics and economic feasibility, but also plays a key role in substantially improving the material's conductivity. The utilization of biochar reduces dependence on non-renewable resources, helping to alleviate environmental pollution problems caused by the use of traditional fossil fuels. Therefore, from a practical application perspective, the strategy of using biochar as a carbon coating agent not only provides an innovative approach to optimizing the performance of lithium iron phosphate materials, but also demonstrates great potential in promoting green energy transformation and achieving sustainable development goals. This preparation method uses a secondary calcination process to effectively suppress particle agglomeration, resulting in a more uniform carbon coating layer. The selection of carbon sources with significant differences in carbonization temperature helps to form a continuous, disordered-graphitized carbon layer structure during the secondary carbonization process, enabling lithium iron phosphate materials to exhibit excellent electrochemical performance. Moreover, the carbon source raw materials are mainly derived from waste biomass, offering significant environmental and economic advantages, reducing dependence on non-renewable resources, and demonstrating strong feasibility.

[0016] In the preparation of lithium iron phosphate cathode, a bio-based cellulose was first prepared. Wheat bran was crushed and extracted to obtain wheat bran cellulose. Then, the wheat bran cellulose was activated by alkali, methylated and etherified, and hydroxypropylated and etherified to introduce methyl and hydroxypropyl groups into the cellulose molecule structure, thus obtaining bio-based cellulose. This bio-based cellulose utilizes agricultural waste wheat bran as raw material, reducing raw material costs and providing abundant sources, which aligns with the concept of sustainable development. Furthermore, its molecular structure contains both ether bonds and hydroxyl groups, exhibiting excellent water solubility. The hydroxyl groups on the hydroxypropyl groups can achieve adsorption, and the carbon chains thereon can... This process achieves a coating effect, allowing the particles to be adsorbed and encapsulated during the grinding process, preventing particle aggregation and enabling the particles to be fully refined. This results in the construction of a complete carbon layer on the surface of lithium iron phosphate particles. Furthermore, the methyl and hydroxypropyl groups in the molecular structure gradually release small molecules (methane and propanol) during pyrolysis, forming "through-pore micropores" in the carbon layer. This effectively protects the lithium iron phosphate particles and limits their expansion and pulverization. The micropores also enable rapid migration of lithium ions, ultimately resulting in lithium iron phosphate materials with excellent electrochemical performance, rate performance, and cycle stability. Detailed Implementation

[0017] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1: This embodiment describes a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 60°C for 12 hours to obtain dried wheat bran. Then grind it through a 100-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 100mL of 5% sodium hydroxide solution and stir for 1.5h at 60℃ and 200r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum. Wash the filter cake three times with distilled water and then freeze dry to obtain wheat bran cellulose. Step S3: Add 10g of wheat bran cellulose to 80mL of 20% sodium hydroxide solution and stir for 3h at 30℃ and 200r / min. After the reaction is complete, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step S4: Add 100 mL of alkalized cellulose solution to the reaction vessel, introduce 20 mmol of chloromethane, and react for 4 h at 40 °C and 0.3 MPa. Then add 15 mmol of propylene oxide and continue the reaction for 2 h at 70 °C and 0.6 MPa. After the reaction is completed, cool the reaction product to room temperature, adjust the pH to 6.5 with 8% acetic acid solution, then vacuum filter, wash the filter cake three times with ethanol solution, freeze dry, and pulverize through a 100-mesh sieve to obtain bio-based cellulose. Step S5: Add 3000g lithium carbonate, 752.1g ferric phosphate, 330g carbon source, and 10205.3mL deionized water to a mixer and stir for 20 minutes at 30℃ and 800r / min. Then place the mixture in a sand mill and mill for 1 hour at 1000r / min, followed by milling for 3 hours at 3000r / min to obtain a first-stage milled slurry. The carbon source includes 180g glucose and 150g bio-based cellulose. Step S6: Spray dry the primary sand-milling slurry at an inlet temperature of 170℃ and an outlet temperature of 110℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S7: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 5 hours to obtain primary carbon-coated lithium iron phosphate. Step S8: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of bio-based cellulose and 9545.25mL of deionized water to a sand mill, and mill for 30 minutes at a speed of 1000r / min, and then mill for 1 hour at a speed of 3000r / min to obtain secondary milled slurry. Step S9: Spray dry the secondary sand-milling slurry at an inlet temperature of 170℃ and an outlet temperature of 110℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S10: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 5 hours to obtain secondary carbon-coated lithium iron phosphate. Step S11: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 110℃ for 1 hour. After that, roll and cut it, and then place it in an oven to dry at 110℃ for 10 hours to obtain the lithium iron phosphate cathode.

[0019] Example 2: This embodiment describes a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 80°C for 12 hours to obtain dried wheat bran. Then grind it through a 300-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 150mL of 10% sodium hydroxide solution and stir for 5h at 100℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum, wash the filter cake 5 times with distilled water, and then freeze dry to obtain wheat bran cellulose. Step S3: Add 10g of wheat bran cellulose to 100mL of 30% sodium hydroxide solution and stir for 8h at 40℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step S4: Add 100 mL of alkalized cellulose solution to the reaction vessel, introduce 60 mmol of chloromethane, and react for 6 h at 50 °C and 0.5 MPa. Then add 45 mmol of propylene oxide and continue the reaction for 3 h at 80 °C and 0.8 MPa. After the reaction is completed, cool the reaction product to room temperature, adjust the pH to 7.5 with 10% acetic acid solution, then vacuum filter, wash the filter cake 5 times with ethanol solution, freeze dry, and pulverize through a 300-mesh sieve to obtain bio-based cellulose. Step S5: Add 3000g lithium carbonate, 752.1g ferric phosphate, 330g carbon source, and 10205.3mL deionized water to a mixer and stir for 40 minutes at 60℃ and 1000r / min. Then place the mixture in a sand mill and mill for 2 hours at 1500r / min, followed by milling for 4 hours at 4000r / min to obtain a first-stage milled slurry. The carbon source includes 165g glucose and 165g bio-based cellulose. Step S6: Spray dry the primary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S7: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain primary carbon-coated lithium iron phosphate. Step S8: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of bio-based cellulose and 9545.25mL of deionized water to a sand mill and mill for 50 minutes at a speed of 1500r / min, and then mill for 3 hours at a speed of 4000r / min to obtain secondary milled slurry. Step S9: Spray dry the secondary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S10: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain secondary carbon-coated lithium iron phosphate. Step S11: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 120℃ for 2 hours. After that, roll and cut it, and then place it in an oven to dry at 120℃ for 12 hours to obtain the lithium iron phosphate cathode.

[0020] Example 3: This embodiment describes a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 60°C for 12 hours to obtain dried wheat bran. Then grind it through a 100-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 100mL of 5% sodium hydroxide solution and stir for 1.5h at 60℃ and 200r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum. Wash the filter cake three times with distilled water and then freeze dry to obtain wheat bran cellulose. Step S3: Add 10g of wheat bran cellulose to 80mL of 20% sodium hydroxide solution and stir for 3h at 30℃ and 200r / min. After the reaction is complete, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step S4: Add 100 mL of alkalized cellulose solution to the reaction vessel, introduce 20 mmol of chloromethane, and react for 4 h at 40 °C and 0.3 MPa. Then add 15 mmol of propylene oxide and continue the reaction for 2 h at 70 °C and 0.6 MPa. After the reaction is completed, cool the reaction product to room temperature, adjust the pH to 6.5 with 8% acetic acid solution, then vacuum filter, wash the filter cake three times with ethanol solution, freeze dry, and pulverize through a 100-mesh sieve to obtain bio-based cellulose. Step S5: Add 3000g lithium carbonate, 752.1g ferric phosphate, 330g carbon source, and 10205.3mL deionized water to a mixer and stir for 20 minutes at 30℃ and 800r / min. Then place the mixture in a sand mill and mill for 1 hour at 1000r / min, followed by milling for 3 hours at 3000r / min to obtain a first-stage milled slurry. The carbon source includes 150g glucose and 180g bio-based cellulose. Step S6: Spray dry the primary sand-milling slurry at an inlet temperature of 170℃ and an outlet temperature of 110℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S7: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 5 hours to obtain primary carbon-coated lithium iron phosphate. Step S8: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of bio-based cellulose and 9545.25mL of deionized water to a sand mill, and mill for 30 minutes at a speed of 1000r / min, and then mill for 1 hour at a speed of 3000r / min to obtain secondary milled slurry. Step S9: Spray dry the secondary sand-milling slurry at an inlet temperature of 170℃ and an outlet temperature of 110℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S10: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 5 hours to obtain secondary carbon-coated lithium iron phosphate. Step S11: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 110℃ for 1 hour. After that, roll and cut it, and then place it in an oven to dry at 110℃ for 10 hours to obtain the lithium iron phosphate cathode.

[0021] Example 4: This embodiment describes a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 80°C for 12 hours to obtain dried wheat bran. Then grind it through a 300-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 150mL of 10% sodium hydroxide solution and stir for 5h at 100℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum, wash the filter cake 5 times with distilled water, and then freeze dry to obtain wheat bran cellulose. Step S3: Add 10g of wheat bran cellulose to 100mL of 30% sodium hydroxide solution and stir for 8h at 40℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step S4: Add 100 mL of alkalized cellulose solution to the reaction vessel, introduce 60 mmol of chloromethane, and react for 6 h at 50 °C and 0.5 MPa. Then add 45 mmol of propylene oxide and continue the reaction for 3 h at 80 °C and 0.8 MPa. After the reaction is completed, cool the reaction product to room temperature, adjust the pH to 7.5 with 10% acetic acid solution, then vacuum filter, wash the filter cake 5 times with ethanol solution, freeze dry, and pulverize through a 300-mesh sieve to obtain bio-based cellulose. Step S5: Add 3000g lithium carbonate, 752.1g ferric phosphate, 330g carbon source, and 10205.3mL deionized water to a mixer and stir for 40 minutes at 60℃ and 1000r / min. Then place the mixture in a sand mill and mill for 2 hours at 1500r / min, followed by milling for 4 hours at 4000r / min to obtain a first-stage milled slurry. The carbon source includes 135g glucose and 195g bio-based cellulose. Step S6: Spray dry the primary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S7: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain primary carbon-coated lithium iron phosphate. Step S8: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of bio-based cellulose and 9545.25mL of deionized water to a sand mill and mill for 50 minutes at a speed of 1500r / min, and then mill for 3 hours at a speed of 4000r / min to obtain secondary milled slurry. Step S9: Spray dry the secondary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S10: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain secondary carbon-coated lithium iron phosphate. Step S11: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 120℃ for 2 hours. After that, roll and cut it, and then place it in an oven to dry at 120℃ for 12 hours to obtain the lithium iron phosphate cathode.

[0022] Comparative Example 1: This comparative example illustrates a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 60°C for 12 hours to obtain dried wheat bran. Then grind it through a 100-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 100mL of 5% sodium hydroxide solution and stir for 1.5h at 60℃ and 200r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum. Wash the filter cake three times with distilled water and then freeze dry to obtain wheat bran cellulose. Step S3: Add 10g of wheat bran cellulose to 80mL of 20% sodium hydroxide solution and stir for 3h at 30℃ and 200r / min. After the reaction is complete, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step S4: Add 100 mL of alkalized cellulose solution to the reaction vessel, introduce 20 mmol of chloromethane, and react for 4 h at 40 °C and 0.3 MPa. Then add 15 mmol of propylene oxide and continue the reaction for 2 h at 70 °C and 0.6 MPa. After the reaction is completed, cool the reaction product to room temperature, adjust the pH to 6.5 with 8% acetic acid solution, then vacuum filter, wash the filter cake three times with ethanol solution, freeze dry, and pulverize through a 100-mesh sieve to obtain bio-based cellulose. Step S5: Add 3000g lithium carbonate, 752.1g iron phosphate, 330g bio-based cellulose and 10205.3mL deionized water to a mixer and stir for 20 minutes at 30℃ and 800r / min. Then place it in a sand mill and mill for 1 hour at 1000r / min, and then mill for 3 hours at 3000r / min to obtain a first-stage milled slurry. Step S6: Spray dry the primary sand-milling slurry at an inlet temperature of 170℃ and an outlet temperature of 110℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S7: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 5 hours to obtain primary carbon-coated lithium iron phosphate. Step S8: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of bio-based cellulose and 9545.25mL of deionized water to a sand mill, and mill for 30 minutes at a speed of 1000r / min, and then mill for 1 hour at a speed of 3000r / min to obtain secondary milled slurry. Step S9: Spray dry the secondary sand-milling slurry at an inlet temperature of 170℃ and an outlet temperature of 110℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S10: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 5 hours to obtain secondary carbon-coated lithium iron phosphate. Step S11: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 110℃ for 1 hour. After that, roll and cut it, and then place it in an oven to dry at 110℃ for 10 hours to obtain the lithium iron phosphate cathode.

[0023] Comparative Example 2: This comparative example illustrates a method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 80°C for 12 hours to obtain dried wheat bran. Then grind it through a 300-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 150mL of 10% sodium hydroxide solution and stir for 5h at 100℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum, wash the filter cake 5 times with distilled water, and then freeze dry to obtain wheat bran cellulose. Step S3: Add 10g of wheat bran cellulose to 100mL of 30% sodium hydroxide solution and stir for 8h at 40℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature to obtain an alkalized cellulose solution. Step S4: Add 100 mL of alkalized cellulose solution to the reaction vessel, introduce 60 mmol of chloromethane, and react for 6 h at 50 °C and 0.5 MPa. Then add 45 mmol of propylene oxide and continue the reaction for 3 h at 80 °C and 0.8 MPa. After the reaction is completed, cool the reaction product to room temperature, adjust the pH to 7.5 with 10% acetic acid solution, then vacuum filter, wash the filter cake 5 times with ethanol solution, freeze dry, and pulverize through a 300-mesh sieve to obtain bio-based cellulose. Step S5: Add 3000g lithium carbonate, 752.1g ferric phosphate, 330g glucose and 10205.3mL deionized water to a mixer and stir for 40 minutes at 60℃ and 1000r / min. Then place it in a sand mill and mill for 2 hours at 1500r / min, and then mill for 4 hours at 4000r / min to obtain a first-stage milled slurry. Step S6: Spray dry the primary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S7: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain primary carbon-coated lithium iron phosphate. Step S8: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of bio-based cellulose and 9545.25mL of deionized water to a sand mill and mill for 50 minutes at a speed of 1500r / min, and then mill for 3 hours at a speed of 4000r / min to obtain secondary milled slurry. Step S9: Spray dry the secondary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S10: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain secondary carbon-coated lithium iron phosphate. Step S11: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 120℃ for 2 hours. After that, roll and cut it, and then place it in an oven to dry at 120℃ for 12 hours to obtain the lithium iron phosphate cathode.

[0024] Comparative Example 3: Comparative Example 3 illustrates a method for preparing a lithium iron phosphate cathode, comprising the following steps: Step S1: Place wheat bran in an oven and dry it at 80°C for 12 hours to obtain dried wheat bran. Then grind it through a 300-mesh sieve to obtain wheat bran powder. Step S2: Add 10g of wheat bran powder to 150mL of 10% sodium hydroxide solution and stir for 5h at 100℃ and 500r / min. After the reaction is completed, cool the reaction product to room temperature, then filter under vacuum, wash the filter cake 5 times with distilled water, and then freeze dry to obtain wheat bran cellulose. Step S3: Add 3000g lithium carbonate, 752.1g ferric phosphate, 330g carbon source, and 10205.3mL deionized water to a mixer and stir for 40 minutes at 60℃ and 1000r / min. Then place the mixture in a sand mill and mill for 2 hours at 1500r / min, followed by milling for 4 hours at 4000r / min to obtain a first-stage milled slurry. The carbon source includes 135g glucose and 195g wheat bran cellulose. Step S4: Spray dry the primary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a primary carbon-coated lithium iron phosphate precursor. Step S5: Place the primary carbon-coated lithium iron phosphate precursor in a tube furnace, introduce nitrogen protection, and calcine at 600℃ for 10 hours to obtain primary carbon-coated lithium iron phosphate. Step S6: Add 3818.1g of primary carbon-coated lithium iron phosphate, 80g of wheat bran cellulose and 9545.25mL of deionized water to a sand mill, and mill for 50 minutes at a speed of 1500r / min, and then mill for 3 hours at a speed of 4000r / min to obtain secondary milled slurry. Step S7: Spray dry the secondary sand-milling slurry at an inlet temperature of 180℃ and an outlet temperature of 120℃ to obtain a secondary carbon-coated lithium iron phosphate precursor. Step S8: Place the secondary carbon-coated lithium iron phosphate precursor in a tube furnace, purge it with nitrogen, and calcine it at 600℃ for 10 hours to obtain secondary carbon-coated lithium iron phosphate. Step S9: Mix 800g of secondary carbon-coated lithium iron phosphate, 100g of polyvinylidene fluoride, 100g of acetylene black and 1050g of N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then place it in an oven to dry at 120℃ for 2 hours. After that, roll and cut it, and then place it in an oven to dry at 120℃ for 12 hours to obtain the lithium iron phosphate cathode.

[0025] Performance testing Using lithium iron phosphate cathodes from Examples 1-4 and Comparative Examples 1-3 as cathodes, lithium sheets as counter electrodes, 1M LiPF6 / (EC+DMC+DEC, v / v / v=1:1:1) as electrolyte, and Celgard 2400 microporous membrane as separator, a CR2032 type button half-cell was assembled. The performance of the CR2032 type button half-cell was tested, and the test results are shown in Table 1.

[0026] Table 1. Results of Physicochemical Performance Tests

[0027] Referring to the data in the table above, based on the data from Examples 1-4, it can be seen that the lithium iron phosphate cathode of this application has excellent electrochemical performance. Based on the comparison between Example 1 and Comparative Example 1 and between Example 2 and Comparative Example 2, it can be seen that using a mixture of glucose and bio-based cellulose as a carbon source can significantly improve the chemical stability of the lithium iron phosphate cathode and enhance its rate performance. Based on the comparison between Example 2 and Comparative Example 3, it can be seen that adding bio-based cellulose can significantly improve the chemical stability of the lithium iron phosphate cathode and enhance its rate performance compared to wheat bran cellulose, indicating that the "methyl and hydroxypropyl" groups in its molecular structure play a significant role.

[0028] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0029] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source, characterized in that, Includes the following steps: Step A1: Lithium carbonate, iron phosphate, carbon source, and deionized water are stirred and mixed, and then milled to obtain a first-stage milling slurry; wherein, the carbon source is one of glucose and bio-based cellulose, or a mixture of the two in any proportion; wherein, the molecular structure of bio-based cellulose has both ether bonds and hydroxyl groups, which enables the construction of a complete carbon layer on the surface of lithium iron phosphate particles, and its molecular structure also contains methyl and hydroxypropyl groups, which will gradually release small molecules during pyrolysis, thereby forming "through-pore micropores" in the carbon layer; Step A2: Spray dry the primary sand-milled slurry to obtain a primary carbon-coated lithium iron phosphate precursor; Step A3: Calcine the primary carbon-coated lithium iron phosphate precursor to obtain primary carbon-coated lithium iron phosphate; Step A4: The carbon-coated lithium iron phosphate, bio-based cellulose and deionized water are milled to obtain a secondary milling slurry; Step A5: Spray dry the secondary sand-milled slurry to obtain a secondary carbon-coated lithium iron phosphate precursor; Step A6: Calcining the secondary carbon-coated lithium iron phosphate precursor to obtain secondary carbon-coated lithium iron phosphate; Step A7: Mix the secondary carbon-coated lithium iron phosphate, polyvinylidene fluoride, acetylene black and N-methylpyrrolidone evenly, then coat it onto aluminum foil, and then roll, cut and dry to obtain the lithium iron phosphate cathode.

2. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 1, characterized in that, The ratio of lithium carbonate, iron phosphate, carbon source, and deionized water used in step A1 is 3000g:752.1g:330g:10205.3mL.

3. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 1, characterized in that, In step A4, the ratio of carbon-coated lithium iron phosphate, bio-based cellulose, and deionized water is 3818.1g:80g:9545.25mL.

4. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 1, characterized in that, Step A7: The ratio of the amount of secondary carbon-coated lithium iron phosphate, polyvinylidene fluoride, acetylene black and N-methylpyrrolidone is 800g:100g:100g:1050g.

5. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 1, characterized in that, The bio-based cellulose is prepared by the following steps: Step B1: Dry the wheat bran to obtain dried wheat bran, then grind and sieve it to obtain wheat bran powder; Step B2: Add wheat bran powder to sodium hydroxide solution and stir to react. After the reaction is complete, cool the reaction product, then filter under vacuum, wash the filter cake, and then freeze dry to obtain wheat bran cellulose. Step B3: Add wheat bran cellulose to sodium hydroxide solution and stir to react. After the reaction is complete, cool the reaction product to obtain an alkalized cellulose solution. Step B4: Add the alkalized cellulose solution to the reaction vessel, introduce chloromethane to carry out the reaction, then add propylene oxide to continue the reaction. After the reaction is completed, cool the reaction product, then adjust the pH with acetic acid solution, then vacuum filter, wash the filter cake with ethanol solution, then freeze dry, pulverize and sieve to obtain bio-based cellulose.

6. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 5, characterized in that, In step B2, the ratio of wheat bran powder to sodium hydroxide solution is 10g:100-150mL; the mass fraction of the sodium hydroxide solution is 5-10%.

7. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 5, characterized in that, In step B3, the ratio of wheat bran cellulose to sodium hydroxide solution is 10g:80-100mL; the mass fraction of the sodium hydroxide solution is 20-30%.

8. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 5, characterized in that, In step B4, the ratio of alkalized cellulose solution, chloromethane, and propylene oxide is 100 mL: 20-60 mmol: 15-45 mmol.

9. The method for preparing a lithium iron phosphate cathode using bio-based cellulose as a carbon source according to claim 5, characterized in that, The acetic acid solution in step B4 has a mass fraction of 8-10%.

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